1. Field of the Invention
This invention relates to multiple hearth furnaces and, more particularly, to an improved method and apparatus for introducing gases or liquids into the furnace for purposes of mass transfer.
2. Description of the Prior Art
In many industrial processing applications, it is important to bring about heat transfer and mass transfer at the same time in order to produce the desired product. These types of processes are often carried out in multiple hearth furnaces. For example, activated carbon is produced by heating raw material (carbonaceous material) to high temperatures (1500.degree.-2000.degree. F.) and, while at these temperatures, oxidizing the material with steam, air and carbon dioxide to selectively remove material. This leaves the product with a large internal surface area. The reactivation of spent activated carbon is a similar process. Another example is the reduction of nickel-containing containing ores. In this process, the ores are heated to 1200.degree.-1500.degree. F. and reacted with reducing gases to reduce the nickel oxide in the ore to metallic nickel.
The multiple hearth furnace (MHF) is a useful heat transfer device at the high temperatures required, but contact between the solids and fluids is not satisfactory with present designs because there is little mixing of the same. With vertically arranged furnaces, most of the solid-fluid contact is during a limited time when solids drop from hearth to hearth. The prior art in this regard is discussed in U.S. Pat. No. 5,080,025. In some designs, particularly rotary hearth furnaces, a multiplicity of nozzles or other inlets have been affixed to the furnace shell and jets of gases or other fluids are passed through these nozzles over the bed of solids. Such arrangements may be seen in U.S. Pat. Nos. 506,511; 4,227,873 and 4,391,208. This method provides little mixing of solids and gases. Most of the gases pass over the bed of solids and there is only localized reaction at the surface of the bed. This localized reaction can result in operational problems, such as nonuniform product.
U.S. Pat. Nos. 534,160; 1,825,947; 2,505,363; and 3,671,167 disclose multiple hearth furnaces having apertures in the rabble arms for feeding air into the furnace. However, it would be impractical to pass certain treatment fluids, such as steam, directly through the rabble arms of these designs. The rabble arms are typically fed directly from the hollow center shaft of the furnace. Center shafts and arms are almost uniformly built from cast iron, which would rust if used to convey steam or other corrosive treatment fluids. Additionally, the primary purpose for passing fluids through rabble arms to date has been to cool the arms. Problems and objectives related to improving mass transfer in the furnace have not been addressed. Furthermore, the designs in the above patents fall short of providing the intimate mixing of solids and gases found desirable in the aforementioned processes.
U.S. Pat. No. 488,797 discloses a roasting furnace having rabble arms which also serve as feed pipes for tubes U. Tubes U extend downward behind the rabbles to supply air for oxidizing purposes in the process of calcination. Again, this arm-fed design would not be suitable for conveying steam or corrosive fluids. Additionally, the tubes U are not optimally located for providing maximum solid-gas contact. U.S. Pat. No. 4,906,183 discloses a rotary hearth kiln 10 having a manifold pipe 76 with depending air nozzles 78 to blow materials, such as slate, off a hearth 12 into a receiver 88. The air nozzles 78 are said to aid in temperature reduction of the material under treatment, but they are not designed and placed to accomplish both heat transfer and mass transfer as provided by the present invention.
Therefore, it is an object of the present invention to substantially improve mass transfer in multiple hearth furnaces between fluids and solids in processes such as manufacturing activated carbon, reactivating spent activated carbon and reducing nickel ore to metallic nickel.